The present invention relates to a method of endrounding loose fibres of thermoplastic material for use in brush making, toothbrush making in particular.
Brushes for dental or cosmetical use must have bristles with rounded free ends to avoid injury or harm to the user. Rounding of the bristle ends is done after the bristles have been attached to a brush body by means of abrasive methods or radiation. According to a more recent technology, bristles are implant-molded with the brush body. This technology permits the use of pre-cut and pre-rounded fibre, avoiding a finishing step after the bristles have been attached to the brush body. It is more convenient to process the fibre for the purpose of endrounding before attachment to the brush body than performing the finishing steps with the bristles already attached to the brush body.
When an abrasive endrounding method is used, it is mandatory to have the free ends of the fibre perfectly aligned in a plane perpendicular to the length of the tuft or bundle presented to the working surface of the endrounding tool. This requirement has been recognized and confirmed in the patent literature, for example EP 0 346 646 B1. Unless this requirement is satisfied, the fibre ends are not consistently rounded and smoothened from all sides. Also, the amount of fibre that can be processed in a single step is limited because the fibre ends must be free to be laterally deflected so that the fibre end surface can be presented to and engaged by the working surface of the endrounding tool from all sides and directions.
Recently, a new endrounding technology has been proposed wherein a perfect and consistent rounding quality is achieved by producing friction rather than abrasion between the working surface of the endrounding tool and the free ends of the fibres. As disclosed in endrounding tool and the free ends of the fibres. As disclosed in German Utility Model 296 14 118 the rounding effect achieved with this technology is not completely understood, but relative speed between the fibre ends and the working surface is an essential parameter. Relative speed must produce friction in an amount sufficient to heat the thermoplastic material of the fibre close to its melting point, but excessive heat would destroy the fibre ends. This technology has been successfully used on tufts of bristles attached to toothbrush bodies so that the bristles (or fibres) were exposed to the working surface of the endrounding tool in relatively small tufts. It was considered impossible to process large amounts of bristles or fibres in a bundle because of the expected concentration of friction and heat in the central portion of the plane in which the free ends of the fibres are exposed. Such concentration of friction, and thus heat, in the central portion of the bundle is due to the increased lateral support of the fibres in the central portion against deflection by engagement with the endrounding tool. The fibre in the center portion of the bundle is stiffer because it is supported laterally by surrounding fibre. Therefore, a uniform distribution of heat throughout the cross-section of the bundle cannot be achieved. As mentioned earlier, however, the new endrounding technology requires consistent heating conditions within close limits. A similar rounding technique is disclosed in U.S. Pat. No. 2,554,777 where the fibres are processed while they are mounted on a brush.
The present invention provides a method of endrounding loose fibres of thermoplastic material for use in brushmaking wherein the new endrounding technology relying on friction rather than abrasion can be used and relatively large amounts of fibre can be processed at one time. According to the invention, the fibres are presented to an endrounding tool in a tuft or bundle of relatively large diameter, and the free ends of the fibres are exposed to the working surface of the tool to produce relative movement between the fibre ends and the working surface, causing the material of the fibres to be heated by friction of a predetermined intensity. While the fibres are exposed to the working surface of the tool, they are permitted to flex laterally and are held in an axial direction of the tuft or bundle with their free ends commonly defining a surface different from a plane which is perpendicular to the axial direction. Since the fibres have their ends lying in a plane different from a plane which is perpendicular to the individual fibre can be adjusted in a manner to achieve consistent frictional heating throughout the cross-section of the tuft or bundle. Specifically, in the preferred embodiment the fibres have a cantilever length which is greater close to the centre of the tuft or bundle than closer to its periphery. From a geometrical point of view, the fibres have their free ends located in a surface of convex shape. The convex shape can be conical, frustroconical or hemispherical.